The present invention relates to a watertight and thermally insulating tank, particularly for storing liquefied gases, such as liquefied natural gases with a high methane content, at a temperature of about xe2x88x92160xc2x0 C., said tank being built into a bearing structure of a ship, particularly the hull of a ship intended for transporting liquefied gases by sea.
French Patent Application No. 99/07254 discloses such a watertight and insulating tank built into a bearing structure, particularly of a ship, in the form of a polyhedron, particularly an irregular octahedron, the tank corners of which generally make an angle of 90xc2x0 or 135xc2x0; said tank comprising two successive watertightness barriers, one of them a primary barrier in contact with the product contained in the tank and the other a secondary barrier arranged between the primary barrier and the bearing structure, these two watertightness barriers alternating with two thermally insulating barriers. According to that document, the primary watertightness barrier consists of thin metal sheets, particularly substantially flat strakes made of Invar sheet, mechanically held on the primary insulating barrier via their turned-up longitudinal edges.
The secondary barriers and the primary insulating barrier essentially consist of a collection of prefabricated panels fixed mechanically to the bearing structure but not bonded to it, each panel comprising, in succession, a first rigid plate forming the bottom of the panel, a first layer of thermal insulation carried by said bottom plate and with it constituting a secondary insulating barrier element, a second layer of thermal insulation which partially covers the aforementioned first layer and a second rigid plate forming the cover of the panel and covering the second layer of thermal insulation which with said second plate constitutes a primary insulating barrier element.
Still according to that document, the regions where the primary insulating barrier elements of two adjacent panels meet are filled with insulating tiles each consisting of a layer of thermal insulation covered by a rigid plate, the rigid plates of the insulating tiles and the second rigid plates of the panels constituting a substantially continuous wall capable of supporting the primary watertightness barrier, the regions at the joints between the secondary insulating barrier elements being filled with connectors made of insulating material.
Also known from French patent No. 2 683 786 is a secondary insulating barrier consisting of a number of caissons each of which comprises a parallelepipedal box made of plywood equipped internally with longitudinal and transverse partitions and filled with an insulant in particulate form known by the name of perlite. However, these insulating barriers have a complex structure and are expensive to manufacture.
In order to produce said layers of thermal insulation, it is known practise to use cellular foam, particularly polyurethane foam having, for example, a density of about 105 kg/M3, or cellular foam reinforced, for example, with glass fibers, and having, for example, a density of about 120 kg/m3. The use of said prefabricated panels considerably reduces the time and cost involved in producing the tank.
It is known that when the ship moves about in the swell, the deformation of its hull generates, at the primary and secondary watertightness barriers, very high tensile stresses which add to the tensile stresses generated in these watertightness barriers by the cooling of the tank. As is known, expansion gussets formed by the turned-up longitudinal edges of the Invar strakes allow the primary watertightness barrier to be given limited stretch in its transverse direction, of the order of 0.3 to 0.6 mm per meter so as to elastically absorb the tensile stresses generated by the cooling of the tank and so as to compensate for the corresponding contraction of the strakes.
However, when layers of thermal insulation made of cellular foam are used, these have a tendency, given that they are compressible, to compress and contract substantially perpendicularly toward the walls of the bearing structure, under the action of the static pressure of the contents of the tank, and of the dynamic pressure produced on the walls of the tank by the movements of the liquid during transport, which movements are due to the rolling and pitching of the ship. Such compression and contraction also contributes to generating tension in the primary watertightness barrier, particularly in the transverse direction of the strakes, and particularly near the longitudinal solid angles of intersection of the tank. In a known way, the primary watertightness barrier can be produced using steel sheet elements which have transverse and longitudinal ribs butt-welded together to form a goffered surface. The ribs of such a surface can open up to allow the primary watertightness barrier to stretch. However, such elements exhibit significant movements of thermal expansion and contraction. On the other hand, when substantially flat strakes made of Invar sheet with turned-up longitudinal edges are used in association with a compressible layer of thermal insulation, the thermal contraction movements are of more limited amplitude but there is a risk that the primary watertightness barrier will become damaged under the compression and contraction of the layer of insulation, because they generate transverse tensile forces on the watertightness barrier, the expansion gussets at the turned-up edges of which may prove insufficient to allow a corresponding elongation.
The purpose of the invention is to provide such a tank, the walls of which have prefabricated panels such as the aforementioned ones, but which does not have the aforementioned drawbacks.
For that, the invention provides a watertight and thermally insulating tank built into a bearing structure, particularly of a ship, said bearing structure having a number of substantially flat faces adjacent via their longitudinal edges and having a polygonal cross section, each pair of longitudinally adjacent faces forming a dihedron, said tank comprising two successive watertightness barriers, one of them a primary watertightness barrier in contact with the product contained in the tank, and the other a secondary watertightness barrier arranged between said primary watertightness barrier and the bearing structure, a primary thermally insulating barrier being arranged between these two watertightness barriers and a secondary thermally insulating barrier being arranged between said secondary watertightness barrier and the bearing structure, the secondary insulating and watertightness barriers and the primary insulating barrier being essentially formed of a collection of wall elements juxtaposed on the bearing structure over substantially its entire interior surface, said wall elements being partially deformable in the direction of their thickness, said wall elements being capable of supporting and of retaining the primary watertightness barrier, said primary watertightness barrier having substantially flat running metal strakes made of thin sheet metal with a low coefficient of expansion, the longitudinal edges of which are turned up toward the inside of the tank, each running strake being assembled watertightly with at least one longitudinally adjacent running strake, the adjacent turned-up edges of said running strakes being welded to the two faces of a weld support which is mechanically held on said wall elements, characterized in that said primary watertightness barrier comprises, on each side of the longitudinal solid angle of intersection of at least one of said dihedra, a longitudinal row of corrugated corner strakes, each corner strake having a first longitudinal edge, opposite said solid angle of intersection of the dihedron, which is turned up toward the inside of the tank and welded to one face of a weld support held mechanically on said wall elements, the longitudinal edge of a running strake longitudinally adjacent to said corner strake being welded to the other face of said weld support, each corner strake comprising at least one corrugation between its two longitudinal edges so as to be capable of deforming transversely to elastically follow any deformation there might be of said wall elements supporting said primary watertightness barrier, it being possible for said deformation to be brought about by the static or dynamic pressure of the product contained in said tank and/or the thermal contraction.
As a preference, each corner strake has several, preferably three, corrugations of substantially the same height or of the same height.
Advantageously, the primary watertightness barrier comprises, at said solid angle of intersection of the dihedron, a metal angle bracket, the angle of which is substantially equal to the angle of said dihedron, each corner strake having its second longitudinal edge welded to said metal angle bracket.
As a preference, said wall elements comprise, on their opposite face to said bearing structure, support plates forming a substantially continuous wall; each leg of said angle bracket being fixed to at least one of said support plates by at least one fixing screw engaged through an oblong hole in said leg and fixed into said support plate, said oblong hole being substantially perpendicular to said solid angle of intersection of the dihedron so as to offer said leg limited freedom of movement in this direction with respect to said support plate; each oblong hole being covered by a corner strake, one longitudinal edge of which is fixed to said leg between the solid angle of intersection of said angle bracket and said oblong hole.
According to another feature of the invention, said wall elements comprise, along said solid angle of intersection of the dihedron, prefabricated corner structures, each corner structure comprising two substructures designed and arranged substantially symmetrically with respect to the plane that bisects said dihedron, each of said substructures having, in succession, through its thickness: a first rigid plate forming the bottom of the substructure, fixed mechanically and/or bonded to said bearing structure, a first layer of thermal insulation carried by said bottom plate, a second rigid plate covering substantially the entirety of said first layer to provide, with it and said bottom plate, a secondary insulating barrier element, a secondary watertightness barrier element bonded onto said second plate, a second layer of thermal insulation which partially covers said second plate, forming thereon a border not covered by said second layer, and a third rigid plate forming said support plate of the substructure and covering the second layer of thermal insulation to provide, with it, a primary insulating barrier element; the respective bottom plates of said substructures being respectively substantially parallel to the two faces of said dihedron.
As a preference, the two legs of said angle bracket are fixed respectively to the support plates of said two substructures.
Advantageously, a rigid thrust plate is inserted between the secondary insulating barrier elements of said two substructures substantially in said plane that bisects the dihedron, said secondary insulating barrier elements of the two substructures each having a longitudinal face substantially parallel to said bisecting plane and bearing against said thrust plate.
As a preference, the secondary insulating barrier elements of the two substructures of each corner structure have a facet cut substantially at right angles to said bisecting plane so as to define an empty space between said corner structure and the solid angle of intersection of the dihedron of the bearing structure, a sheet of tension-resistant insulating material covering said cut facet in order to hold said two substructures together.
Advantageously, each corner structure comprises a continuous, gas tight and liquid tight flexible web, preferably comprising a continuous deformable thin sheet of aluminum interposed between two sheets of glass fabric, two border parts of which are respectively fixed watertightly to the secondary watertightness barrier elements of the two substructures, a central part of said web, which passes through said bisecting plane not being fixed to said substructures, so as to allow it to adopt a variable curvature as corner structures deform in said way.
As a preference, a corner gasket made of flexible insulating material is inserted between the primary insulating barrier elements of said two substructures and on said web, said corner gasket not being fixed to said web.
Advantageously, the bearing structure comprises metal flats welded to its internal surface parallel to said solid angle of intersection of the dihedron and on each side thereof, the bottom plate of each substructure of a corner structure being positioned between said solid angle of intersection of the dihedron and one of said flats; a corner structure being fixed to the bearing structure using studs welded substantially perpendicularly to the internal surface of the bearing structure, said studs each having their free end threaded, the studs being arranged in such a way that the studs lie between said solid angle of intersection of the dihedron and said flats, in line with said border not covered with the secondary insulating barrier elements of each substructure, a well being formed in line with each stud through the second plate and the first layer of thermal insulation of a substructure, the bottom of the well being formed by the bottom plate of said substructure and having an elongate orifice to allow a stud to pass, a washer being placed over the stud to rest against the bottom plate, held on by a nut screwed onto said stud, said elongate orifice being oriented substantially at right angles to said solid angle of intersection of the dihedron, said stud being engaged near the end of said elongate orifice away from said solid angle of intersection of the dihedron so as to allow said bottom plate a limited movement with respect to said bearing structure toward said flat, a deformable wad, preferably of curable resin, being inserted between said flat and said bottom plate.
According to yet another feature of the invention, said wall elements comprise prefabricated panels, each panel comprising, in succession, through its thickness: a first rigid sheet forming the bottom of the panel, fixed mechanically and/or bonded to said bearing structure, a first layer of thermal insulation carried by said bottom plate to provide, with it, a secondary insulating barrier element, a second layer of thermal insulation which partially covers said first layer, forming thereon a border not covered by said second layer, and a second rigid sheet forming said support plate of the panel and covering the secondary layer of thermal insulation to provide, with it, a primary insulating barrier element.
As a preference, said wall elements also comprise insulating tiles, each comprising a layer of thermal insulation covered by a rigid plate forming said support plate for the insulating tile, at least one of said insulating tiles being bonded in each region where the primary insulating barrier element of a substructure of the corner structure meets the primary insulating barrier element of a panel adjacent to said corner structure, so as to fill the region of this joint.
Advantageously, the angle of said dihedron is greater than 90xc2x0, and preferably substantially equal to 135xc2x0.